This invention relates to a circuit for powering fluorescent lamps and more particularly to a ballast circuit for extending the life of one or more lamps attached thereto.
Fluorescent lamps are in wide spread use in office buildings, public areas and mass transit vehicles throughout the world. Periodically, these fluorescent lamps require servicing. For example, a filament may have broken or sputtered away, or a lamp may have become unseated from its socket. With conventional ballast arrangements, these circumstances create a potential fire or shock hazard, especially when service personnel remove or replace one lamp of several lamps connected to the same ballast arrangement. Accordingly, it would be desirable to provide a ballast arrangement in which fault conditions are accommodated by reducing or eliminating any shock and fire hazards.
While fluorescent or discharge lamps offer significantly greater efficiency of light output in terms of lumens per watt, as compared to incandescent lamps, their effective lifespans are adversely affected by sputtering of their filaments which may occur whenever a cold start is effected. It has been empirically determined that the life of a fluorescent lamp is dependent, in part, upon the number of times that the lamp is switched on and off. As a general rule, it has been observed that approximately three hours of lamp life is dissipated each time that a lamp is switched on using a conventional cold start ballast arrangement. A fluorescent lamp will last for a number of years if it is switched on and left on.
One approach to maintain the filaments warm and thereby reduce sputtering effects is in a ballast arrangement in which an impedance is selectively introduced into the lamp circuit to reduce the excitation voltage below that necessary to sustain the arc discharge and thereby to turn the lamp off while maintaining current through the filament. Such prior art circuits are merely voltage reducing circuits which selectively terminate light production from the lamps from the high-voltage output side of the ballast arrangement. Risks of shock to maintenance personnel, as well as the risk of fire, remain. There is therefore a need for a simple ballast circuit capable of reducing or eliminating such fire and shock hazards. Further, there remains a need to reduce filament sputtering when the lamp is initially turned on.
In accordance with the invention, several lamps may be electrically connected to a single ballast unit, each of which has its arc voltage turned off independent of its filament voltage when a fault or interrupt condition is detected, and has the arc voltage applied to the lamp filaments only when the lamp filaments are warm or after a time period during which they are warmed so that sputtering when an excitation for arc voltage is subsequently applied is reduced.